FIG. 5 shows an example of a conventional small-sized reduction gear of this type, which employs a planetary gear mechanism. In FIG. 5, a sun gear 103 is fitted onto a motor shaft 102 which is at the center of a small-sized motor 101. A lower case 104 is threadedly connected with the small-sized motor 101, with the motor shaft 102 serving as a common central axis. The lower case 104 is equipped with an internal gear 105, and two to four planet gears 106 are provided between the internal gear 105 and the sun gear 103. The plurality of planetary gears 106 mesh with the sun gear 103 and mesh with the internal gear 105. The centers of the plurality of planet gears 106 are connected by a holder member 107, and an output shaft gear 109 is provided at the center of the holder member 107. An intermediate carrier member 108 is connected to the lower case 104 and constitutes a bearing for the output shaft gear 109. The output shaft gear 109 is in mesh with the planet gears 106, which are in mesh with the internal gear 105. The gear at the center provides a driving force, which is transmitted through the planet gears 106 to the holder member 107 so as to rotate the same. The components which are thus operatively connected constitute a unit. When a high reduction ratio is to be obtained, a number of such units are stacked one upon another. In the last stage of this stacked structure, an output shaft 110 exists at the center of rotation of the holder member 107.
Further, Japanese Patent Laid-Open No.1-316545 and 1-316546, filed by the present inventors, disclose small-sized reduction gears using a planetary gear mechanism. These reduction gears provide an improvement over the above-described conventional reduction gear.
In small-sized reduction gears using a planetary gear mechanism having such a conventional construction, speed reduction is effected in the course of transmission from the sun gear 103 to the holder member 107 through the planet gears 106. In such reduction gears, the reduction ratio effected by one planetary gear unit is small, so that a large number of units must be stacked together before a large reduction ratio can be obtained. Since a large number of gears are thus combined to effect speed reduction, a great number of gear parts have to be used, resulting in high production costs. Further, this involves a high noise level, as well as a poor transmission efficiency and much backlash due to the mesh of a large number of gears. The small-sized reduction gears according to Japanese Patent Laid-Open No. 1-316545 and 1-316546 have other problems. For example, they have a low rupture strength against external forces, and are liable to be disabled from functioning properly when deformed by external forces.
The present invention aims to solve these problems in the prior art. It is an object of the present invention to attain a reduction in the number of parts and production costs and to provide, at low costs, a highly strong small-sized reduction gear which is small and light in weight, which involves low noise, which provides a high reduction ratio and a high level of transmission efficiency, and which offers high rupture strength despite its small size.